Field of the Invention
The present invention relates to a power transmission system of a hybrid electric vehicle and, more particularly, to a power transmission system of a hybrid electric vehicle which reduces an electrical load by increasing the portion of a mechanical power transfer path in splitting engine power to use a large amount of engine power, which replaces an engine mode when a vehicle starts to reduce the number of mode conversions, and which minimizes a change in RPM of every rotation element in mode conversion.
Description of Related Art
Eco-friendly technology of vehicles is a core technology reflecting the survival of automobile industry in the future, and advanced automobile manufacturers direct all their energy to development of eco-friendly vehicles to achieve environmental and fuel efficiency regulation.
Automobile manufacturers have developed an electric vehicle (EV), a hybrid electric vehicle (HEV), a fuel cell electric vehicle (FCEV), and the like, as future automobile technology.
There are limits and restrictions in technology such as weight, cost, and the like, on the future automobiles, and thus, automobile manufacturers have noted hybrid vehicles as alternatives for practical problems to meet the emission gas control and improve fuel efficiency and have fiercely competed to commercialize hybrid vehicles.
Hybrid vehicles use two or more power sources and may be combined in various ways. As power source, an existing gasoline engine or a diesel engine using fossil fuel and a motor/generator driven by electric energy are used together.
Hybrid vehicles use a motor/generator having relatively good low-speed torque characteristics as a primary power source at a low speed and an engine having relatively good high-speed torque characteristics as a primary power source at a high speed.
Thus, in a low-speed section, an operation of an engine using fossil fuel in hybrid vehicles is stopped and a motor/generator is used instead, improving fuel efficiency and reducing exhaust gas.
Power transmission systems of such hybrid vehicles are classified as a single mode type and a multi-mode type.
The mono-mode type power transmission system is advantageous in that it does not require a torque transmission mechanism such as a clutch or a brake, but efficiency is degraded when a vehicle runs at a high speed, having low fuel efficiency, and in order to be applied to a large vehicle, an additional torque amplifier is required.
The multi-mode type power transmission system has high efficiency when a vehicle runs at a high speed, and since it can be designed to amplify torque by itself, the multi-mode type power transmission system is applicable to medium and large-sized vehicles.
Thus, recently, the multi-mode type power transmission system, rather than the mono-mode power transmission system, is largely employed and research into the multi-mode type power transmission system is actively conducted.
The multi-mode type power transmission system includes a plurality of planetary gear sets, a plurality of motor/generators used as motors and generators, a plurality of torque transmission mechanisms (friction components) that control rotation elements of the planetary gear sets, a battery used as a power source of the motor/generators, and the like.
The multi-mode type power transmission system has different operational mechanisms according to connection configurations of the planetary gear sets, the motor/generators, and the torque transmission mechanisms.
Also, the multi-mode type power transmission systems have characteristics in that durability, power transmission efficiency, size, and the like, thereof are varied according to connection configurations, and thus, research and development continues to implement a firmer, compact power transmission system without power loss in the field of power transmission systems of hybrid electric vehicles.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.